The present invention relates to a method and device for the removal of ambiguity in distance, applied especially to a frequency-shift keying continuous-wave (FSK-CW) radar. It can be applied for example to automobile radars and more generally to frequency-shift keying radars that require high detection efficiency while meeting a goal by which they can be manufactured at low cost.
A frequency-shift keying continuous-wave radar can emit in several transmission frequencies, for example in four frequencies. A radar of this kind may be fitted for example into an automobile for an ACC (automotive cruise control) type of function. A radar of this kind is designed to enable control of the cruising speed of automobile vehicles. It detects and localizes the closest vehicle located in the path of the carrier vehicle. The information, especially on distance, is transmitted for example to a computer which, through appropriate interfaces, acts on the engine control system so as to regulate the distance between the carrier vehicle and its predecessor.
Given that, for a given frequency of transmission, the radar is required to make reception during said transmission, one problem to be resolved is that of being sure that a received signal truly corresponds to this given frequency of transmission. This problem to be resolved can be likened to the resolution of second-trace or higher-trace echoes for a pulse radar. In the case of application to an automobile radars this problem is a very big one inasmuch as the potential targets of the radar have radar equivalent surfaces (RES) that are very different from one another. For, it is necessary to prevent for example a truck with a very great RES that is, at the same time, at a great distance from the carrier vehicle from being perceived as a small vehicle at very small distance, with a low RES.
One method could be used to resolve this problem. This method plays on the period of repetition of the frequency frames. To be efficient, this method makes it necessary to vary the period of repetition in very great proportions. This, of itself, leads to low duty factors for the transmitted waves. Now, for a given mean power, a low duty factor means that a high peak power has to be transmitted. This is not possible especially for automobile radars designed for speed control where, for example, firstly it is sought to transmit all the time and as long as possible for constraints of inter-radar interference and, secondly, the prospect of using so-called MMIC technology itself goes against high peak power values.